A light emitting diode (LED) emits visible light, when electric current passes therethrough. The light emitted by a light emitting diode is not particularly bright. Typically the light is monochromatic, i.e. one light emitting diode emits one certain wavelength. The wavelengths emitted by light emitting diodes can vary from red, 700 nm, to blue-violet, 400 nm. Some light emitting diodes may emit in the infrared range, in which case the wavelength emitted by them can be 830 nm or more.
A pn-junction of a light emitting diode emits light, when electric current passes through the light emitting diode. The light emitting diode is composed of a p-type and an n-type semiconductor. Between the semiconductors, there is a so-called pn-junction, where the p-side has a negative charge and the electron-free holes serve as charge carriers, and the n-side has a positive charge and the free electrons serve as charge carriers. When a forward current is induced in a light emitting diode, so that the p-side is arranged at a high potential, and the n-side is arranged at a low potential, electrons flow to the pn-junction area from the n-side, and holes from the p-side. The free holes and electrons are annihilated, i.e. the electrons fill the free holes. This kind of transfer of electrons from a high-energy state to a lower-energy state releases energy. In a light emitting diode, the energy is released in the form of visible light.
Light emitting diodes have a low power consumption, and they are efficient in producing light energy. In addition, light emitting diodes have a long life. Therefore they are used in several practical applications, such as for example self-illuminating displays, number and dial plates, clocks, electronic calculators, speed displays of cars and signal lights. The pn-junction area of a light emitting diode can be wide, and it can be designed according to the application in question. The surface part to be arranged on top of a light emitting diode installed in a device must be made of a light permeable material, so that the light energy emitted by the light emitting diode permeates the surface part of the device and can be observed.
Light emitting diodes are sensitive to electrostatic discharge (ESD). Typically electrostatic discharge occurs when two different materials, one of which has a positive charge and the other has a negative charge, are set in mutual contact. The positively charged material has an electrostatic charge. When this type of electrostatic charge gets into contact with a certain other material, the charge is transferred, and there is created electrostatic discharge.
A remarkable quantity of thermal energy is released in an electrostatic discharge. If the electrostatic charge is discharged in a sensitive electric device, the heat released in the device during the discharge may melt, vaporize or otherwise damage the sensitive components of the device. An electrostatic discharge can damage the device so that while the device still works, in some of its elements or functions, there occur errors or irregularities deviant from the normal operation.
These kind of hidden effects are extremely difficult to observe, and they remarkably shorten the working life of the device. Many electronic devices are sensitive even to low-voltage electrostatic discharge. Therefore manufacturers attempt to avoid electrostatic discharge during the whole production process: during the manufacturing, testing, transport and processing steps. In addition, the products can be subjected to electrostatic discharge when they are being used, which means that the shielding of sensitive products must be taken into account already at the planning stage.
Sensitive electronic products, devices and components are typically packed in materials that shield the products against harmful charges. A product can be shielded mechanically by insulating it against possible external charges. Typically the insulation is carried out by leaving an insulation clearance between the product and the shielding element, said clearance being for example an insulating clearance of air. In practice, the product is put for instance in a thick plastic bag, so that an insulating layer of air is arranged between the product and the bag. This kind of insulation is generally not suited for products during their use, because the cover and the insulating layer may disturb the use or make it cumbersome, or it may even prevent some functions from being performed.
Another generally used shielding method is a metal box installed around the component to be shielded. A metal box provides a good and reliable shielding against electrostatic discharge. The same metal box can typically be used for other types of shielding, for example as an electromagnetic shielding. Metal boxes used for shielding are fairly heavy and expensive. In addition, metal boxes take up a lot of space, wherefore particularly in small devices their size and weight may turn out to be decisive factors. The installation of metal boxes must be planned as one separate step in the assembly process. The installation is precise work and makes the assembly work more difficult. Moreover, a metal box that is reliable as such, is not a feasible shielding for light emitting diodes, because the light emitted by a light emitting diode cannot permeate a metal box. Thus the metal box also insulates the light emitted by the light emitting diode, so that the light emitting diode becomes useless for illumination. Light emitting diodes are particularly sensitive to electrostatic discharge that occurs generally in the normal use of devices containing light emitting diodes.